Re: [RFC PATCH -next 1/2] mm/mglru: use mem_cgroup_iter for global reclaim

[Johannes Weiner]

On Thu, Dec 04, 2025 at 12:31:23PM +0000, Chen Ridong wrote:
> From: Chen Ridong <chenridong@xxxxxxxxxx>
> 
> The memcg LRU was originally introduced for global reclaim to enhance
> scalability. However, its implementation complexity has led to performance
> regressions when dealing with a large number of memory cgroups [1].
> 
> As suggested by Johannes [1], this patch adopts mem_cgroup_iter with
> cookie-based iteration for global reclaim, aligning with the approach
> already used in shrink_node_memcgs. This simplification removes the
> dedicated memcg LRU tracking while maintaining the core functionality.
> 
> It performed a stress test based on Zhao Yu's methodology [2] on a
> 1 TB, 4-node NUMA system. The results are summarized below:
> 
> 					memcg LRU    memcg iter
> stddev(pgsteal) / mean(pgsteal)            91.2%         75.7%
> sum(pgsteal) / sum(requested)             216.4%        230.5%
> 
> The new implementation demonstrates a significant improvement in
> fairness, reducing the standard deviation relative to the mean by
> 15.5 percentage points. While the reclaim accuracy shows a slight
> increase in overscan (from 85086871 to 90633890, 6.5%).
> 
> The primary benefits of this change are:
> 1. Simplified codebase by removing custom memcg LRU infrastructure
> 2. Improved fairness in memory reclaim across multiple cgroups
> 3. Better performance when creating many memory cgroups
> 
> [1] https://lore.kernel.org/r/20251126171513.GC135004@xxxxxxxxxxx
> [2] https://lore.kernel.org/r/20221222041905.2431096-7-yuzhao@xxxxxxxxxx
> Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

The diff and the test results look good to me. Comparing the resulting
shrink_many() with shrink_node_memcgs(), this also looks like a great
step towards maintainability and unification.

Thanks!